Pyrolysis is the chemical decomposition of a condensed substance by heating. The word is coined from the Greek-derived elements pyro—meaning “fire”—and lysis—meaning “decomposition.” Pyrolysis is a special case of thermolysis, and is most commonly used for organic materials. It occurs spontaneously at high temperatures (above 300° C. for wood), for example in wildfires or when vegetation comes into contact with lava in volcanic eruptions. Pyrolysis generally does not involve reactions with oxygen, but can occur in the presence of oxygen. Extreme pyrolysis, which leaves only carbon as the residue, is called carbonization and is also related to the chemical process of charring.
“Pyrolysis oil,” also known as “bio-oil,” is a synthetic fuel under investigation as substitute for petroleum. Generally speaking, pyrolysis has three main products which include bio-oil, char and various non-condensable gases (H2, CO, CO2, CH4). The char and non-condensable gases may be recovered and burned to supply energy to the system, but the condensable gases are rapidly cooled to form condensate droplets that can then be separated from the non-condensable gases due to the substantial difference in boiling points. The composition of two exemplary bio-fuels produced by pyrolysis is shown below:
WhiteSprucePoplarMoisture content, wt %7.03.3Particle size, μm (max)1000590Temperature500497Apparent residence time0.650.48Product Yields, wt %, m.f.Water11.612.2Gas7.810.8Bio-char12.27.7Bio-oil66.565.7Bio-oil composition, wt %, m.f.Saccharides3.32.4Anhydrosugars6.56.8Aldehydes10.114.0Furans0.35—Ketones1.241.4Alcohols2.01.2Carboxylic acids11.08.5Water-Soluble-Total Above34.534.3Pyrolytic Lignin20.616.2Unaccounted fraction11.415.2Source: Piskorz, J., et al. In Pyrolysis Oils from Biomass, Soltes, E. J., Milne, T. A., Eds., ACS Symposium Series 376, 1988.
Since pyrolysis is endothermic, heat transfer considerations dominate the design of pyrolysis reactors. Direct heat transfer with a hot gas (which can be recycled) is possible in a pyrolysis reactor, but it is difficult to provide enough heat with reasonable gas flow-rates due to the low specific heat of a relatively thin gas. Indirect heat transfer with exchange surfaces (such as vessel walls) is also possible, but it is difficult to achieve good heat transfer on both sides of the heat exchange surface, and these methods are not easily scaled up.
Another method of providing heat is direct heat transfer with a circulating hot solid. This is an effective method, but the technology for moving, recovering, and reheating the solid can be complex. Auger technology employs hot sand (for example) and biomass particles that are fed into one end of a screw. The screw mixes the sand and biomass and conveys them along the screw length. It provides a good control of the biomass residence time and does not dilute the pyrolysis products with a carrier or fluidizing gas. However, the sand must be reheated in a separate vessel, and mechanical reliability is a concern. Thus, there has been no large-scale commercial implementation of the auger technology.
Rotating cones have also been used to introduce and mix hot solids with the comminuted biomass. Pre-heated hot sand and biomass particles are introduced into a rotating cone, and the mixture of sand and biomass is transported across the cone surface by centrifugal force due to the rotation. Like other shallow transported-bed reactors, relatively fine particles are required to obtain a good liquid yield, and there have been no large scale commercial implementation of this technology.
Fluidized beds have also been used, whereby biomass particles are introduced into a bed of hot sand fluidized by a gas, which is usually a recirculated product gas. High heat transfer rates from fluidized sand result in rapid heating of biomass particles. There is some ablation by attrition with the sand particles, but it is not as effective as in the ablative processes (not described herein). Heat is usually provided by heat exchanger tubes through which hot combustion gas flows. There is some dilution of the products, which makes it more difficult to condense and then remove the bio-oil mist from the gas exiting the condensers. The main challenges have been in improving the quality and consistency of the bio-oil, but this process has been scaled up by companies such as Dynamotive and Agri-Therm.
Circulating fluidized beds are yet another possibility. Biomass particles are introduced into a circulating fluidized bed of hot sand. Gas, sand and biomass particles move together, with the transport gas usually being a recirculated product gas. High heat transfer rates from sand ensure rapid heating of biomass particles and ablation is stronger than with regular fluidized beds. A fast separator separates the product gases and vapors from the sand and char particles. The sand particles are reheated in fluidized burner vessel and recycled to the reactor. Although this process can be scaled up, it is rather complex and the products are much diluted, which greatly complicates the recovery of the liquid products.
What is needed in the art is a method for pyrolizing biomass with higher conversion rates at lower temperatures, that can be scaled up and is cost effective to perform. Embodiments of the invention, which combines the biomass with a refinery feedstock prior to pyrolysis, meets these needs.